U.S. Pat. Nos. 5,998,925 and 6,409,938 disclose white light emitting semiconductor elements. White light emitted from these elements is mainly produced from a combination of a blue light emitting semiconductor element with a phosphor coating comprising yellow light emitting phosphor YAG of yttrium aluminum garnet. These white light emitting semiconductor elements are disadvantageous in that the color rendering properties of produced white light are unsatisfactory and, thus, the color rendering index Ra is as low as less than 80. Further, white light having a color temperature above 6000 K produced in these elements is very cold light. This is particularly attributable to the absence of a further color component. In these elements, white light having a color temperature below 5000 K cannot be provided at all.
DE10026435 A1 discloses a phosphor in which simultaneous use of YAG as a yellow light emitting component in a phosphor mixture can realize the conversion of a blue or ultraviolet primary radiation to green light and, at the same time, can increase the color rendering index Ra to about 81.
In DE10026435 A1 and WO00/33389 and WO00/33390, the principle of RGB is applied to produce white light having good quality through a combination of a blue light emission band, a green light emission band and a red light emission band. Further, based on this principle, a yellow component may be used instead of the red component to produce a white light emitting semiconductor element which emits white light with a color rendering index Ra of 81 to 87. In this case, in the spectrum, there is substantially no contribution of the red component to a frequency range.
U.S. Pat. No. 6,084,250 discloses white LED that basically emits an ultraviolet primary radiation and, by virtue of use of a certain phosphor mixture, can realize a color rendering index Ra up to 90 based on the principle of RGB. In this phosphor mixture, in addition to a blue light emitting phosphor BAM comprising a divalent europium-activated barium magnesium aluminate, a sulfide- or Eu(III)-activated red light emitting phosphor is mainly used.
Likewise, U.S. Pat. No. 6,255,670 discloses a blue component, a green component, and a narrow band red component for the production of white light.
In all of these teachings, when the principle of RGB known from television technology and conventional lighting is used and, further, the spectrum is supplemented with a blue-green component, the color rendering index Ra is disadvantageously as low as 89 to 90 at the highest because necessary broadband red component and other light emitting components are absent. When the principle of RGB and a narrow band Eu(III)-activated red component having a line structure in the range of about 610 nm to 625 nm are used, as a general rule, any better white light cannot be produced. It is also known that the sulfide phosphor does not have necessary long-term stability and, thus, the amount of a light emitting flux in the semiconductor element rapidly decreases with the elapse of lighting time, resulting in lowered service life of LED.
It is an object of the invention to eliminate all of these drawbacks and to provide phosphors suitable for use as means for converting ultraviolet and blue radiations to white light in a primary light emitting element that emit light at 300 nm to 500 nm and to provide a phosphor mixture composed mainly of them. The phosphors should produce light having color rendering properties close to those in natural light or incandescent lamp light, and a color rendering index Ra above 90 and color rendering Ia according to CRI standard should be satisfied.
It is another object of the invention to provide an optical device that can convert ultraviolet and blue radiations to white light in a primary light emitting element which emits light at 300 nm to 500 nm and, at the same time, can emit a visible white radiation having a high level of color rendering properties.